1. Technical Field
The present invention generally relates to access networks and more precisely to Passive Optical Networks (PON).
It finds applications, in particular, in Ethernet Passive Optical Networks (EPON) for point to multi-point communications between a terminal and a plurality of units.
2. Related Art
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Automatic line protection can be implemented in telecommunication networks as a mean to dynamically provide an alternative path between two end points in case of a link failure.
Generally, the two end points of a network segment are connected via two independent links, only one of which being active at a given time. The active link is called working link in what follows. The other link is called back-up link and is kept idle as long as the working link is operational.
Upon detection of a failure or of degradation on the working link, both end points synchronize a switch operation from the working link to the back-up link. For that purpose, both end points can run a given protocol in order to:                share and confirm the failure or degradation detection;        carry out a switch operation between the working link and the back-up link;        confirm that the switch operation is effective.        
Upon successful switch operation, the back-up link becomes the working link and the working link is put to an idle state in order to perform a restoration operation.
Usually, the failure or degradation criteria include the following conditions:                signal failure such as loss of signal, loss of frame, bit error rate that is significantly higher than a given higher threshold;        signal degradation: bit error rate greater than a given lower threshold.        
Where usually, line protection is applied to point-to-point network segments, protection in Passive Optical Networks (PON) takes a different shape due to the topology of the PON.
A PON is a single, shared optical fiber that uses inexpensive optical splitters to divide the single fiber from a Central Office (CO) into separate strands feeding individual subscribers. In such networks, information is carried by laser bursts. PONs are called ‘passive’ because there are no active electronics within the access network, except at subscriber endpoints and at the CO. The single fiber is divided by a passive splitter. The architecture of a PON is called a PON tree in what follows.
Ethernet Passive Optical Network (EPON) is based on Ethernet standard, unlike other PON technologies, which are based on Asynchronous Transfer Mode (ATM) standard. EPON enables to utilize the economies-of-scale of Ethernet and provides simple and easy-to-manage connectivity to Ethernet-based IP (for ‘Internet Protocol’) equipment, both at the subscriber endpoints and at the CO.
In such networks, the information is exchanged between layers on a per packet basis. Each packet received in a given layer is encoded with a set of encoding parameters specific to this layer. These parameters should be given through network administration means. A Data Link layer is in charge of sharing the physical resource between the subscriber endpoints and the CO. The Data Link layer is composed by two sub-layers namely the Logical Link (LL) layer and the Medium Access Control (MAC) layer. A Physical layer translates logical communications requests from the Data Link layer into hardware-specific operations to affect transmission or reception of electronic signals.
The IEEE 802.3ah EPON specification, which is also called Gigabit EPON (GEPON), defines Multi-Point Control Protocol (MPCP), Point-to-Point Emulation (P2PE) and Physical layer for 1 Gigabit EPON system (meaning that 1 Gigabit of data is transmitted in the network per second). The IEEE 802.3av specification defines extensions (mainly concerning the Physical layer) for 10 Gigabit EPON. At least, the Standard for Service Interoperability in Ethernet Passive Optical Networks (SIEPON) group, also referenced P1904.1, describes system-level requirements needed to ensure service-level, multi-vendor interoperability of EPON equipment. These specifications complement the existing IEEE Standard 802.3 and IEEE Standard 802.1, which ensure the interoperability at the Physical layer and the Data Link layer.
An EPON network usually includes an Optical Line Terminal (OLT), which can be included in the CO, and one or more Optical Network Unit (ONU), which can be in charge of one or more subscribers of the EPON. The number of ONU managed by each OLT is between four and sixty-four in current deployments.
To control a Point-to-Multi-Point (P2MP) fiber network, EPON uses the MPCP. MPCP performs bandwidths assignment, bandwidth polling, auto-discovery and ranging. MPCP is implemented in the MAC layer, introducing the 64-byte Ethernet control messages:                GATE and REPORT messages are used to assign and request bandwidth;        REGISTER message is used to control auto-discovery process.        
The MAC layer is in charge of transmission arbitration that is allowing a given ONU to enable transmission from its peer for a predetermined interval of time (also called transmission window or timeslot). Start and length of the transmission windows dedicated to each ONU are defined by a Dynamic Bandwidth Allocation (DBA) scheduler comprised in the OLT.
GATE message is sent from the OLT to a given ONU and is used to assign one or several transmission window to that ONU.
REPORT message is a feedback mechanism used by an ONU to indicate its buffer occupancy (meaning the length of a queue of waiting data packets to be sent by the ONU) to the OLT, so that the DBA scheduler can define transmission windows that are adapted to the buffer occupancies of the ONUs.
The IEEE SIEPON defines two types of optical line protection schemes: trunk protection and tree protection.
Referring to FIG. 1, there is shown a trunk protection scheme in a PON.
A working OLT 10.1 is connected to a plurality of n ONUs 12.1-12.n via a passive optical splitter 11. The working OLT 10.1 is connected to the passive optical splitter 11 via a first network segment 13.1. A back-up OLT 10.2 is also connected to the plurality of ONUs 12.1-12.n via the passive optical splitter 11. The back-up OLT 10.2 is connected to the passive optical splitter 11 via a second network segment 13.2.
The passive optical splitter 11 is connected to each of the ONUs 12.1-12.n via one of the network segments 14.1-14.n. 
Thus, the passive optical splitter 11 is arranged to split the first and second network segments 13.1 and 13.2 into n network segments 12.1-12.n. Thus, the single passive optical splitter 11 can be used in the trunk protection scheme.
According to the trunk protection scheme, only the working OLT 10.1 is protected against failure, as well as the first network segment 13.1 spanning from the working OLT 10.1 to the passive optical splitter 11. The first and second network segments 13.1-13.2 together represent the trunk of a PON tree.
Referring now to FIG. 2, there is shown a tree protection scheme in a PON.
A primary OLT 20.1 having the same role as the working OLT 10.1 in the trunk protection scheme, is connected to a plurality of n working ONUs 22.1-22.n via a primary passive optical splitter 21.1. The primary OLT 20.1 is connected to the primary passive optical splitter 21.1 via a first network segment 24.1 and the primary passive optical splitter 21.1 is connected to each of the n working ONUs 22.1-22.n via one of network segments 25.1-25.n. 
The primary passive optical splitter 21.1 is arranged to split the first network segment 24.1 into n network segments 25.1-25.n. 
A secondary OLT 20.2 having the same role as the back-up OLT 10.2 in the trunk protection scheme, is connected to a plurality of n back-up ONUs 23.1-23.n via a secondary passive optical splitter 21.2. The secondary OLT 20.2 is connected to the secondary passive optical splitter 21.2 via a second network segment 24.2 and the secondary passive optical splitter 21.2 is connected to each of the n back-up ONUs 23.1-23.n via one of network segments 26.1-26.n. 
The secondary passive optical splitter 21.2 is arranged to split the second network segment 24.2 into n network segments 26.1-26.n. 
According to the tree protection scheme, the PON tree is fully duplicated. Thus, the primary OLT 20.1 and the n working ONUs 22.1-22.n are protected.
The EPON protocol implements a specific procedure for the OLT to discover the ONUs it is connected to. This specific procedure is called a discovery procedure, during which the OLT determines:                the ONUs connected to the PON tree;        a Round Trip Time (RTT) characterising the optical path between the OLT and each ONU.        
In addition, the OLT imposes a common time reference or clock to all ONUs. The synchronisation of the ONU clock with the OLT clock is maintained during normal operation by time-stamping particular protocol messages that are exchanged at a mandatory minimum frequency so that clock drift cannot occur.
An ONU that has been discovered during a discovery procedure and for which the clock is correctly synchronized with the OLT clock is said to be in a registered state.
An ONU can be deregistered upon explicit decision of the OLT, through a specific protocol procedure, or implicitly upon detection of the following two conditions:                clock desynchronization, i.e. the timestamp indicated by the OLT in the time-stamped protocol messages received by the ONU differs by a too big difference with the timestamp locally maintained in the ONU. This first condition is called “timestamp drift error” condition The “timestamp drift error” condition is defined in the IEEE 802.3 specification, section 5, clause 64.2.1.1 as following: “A condition of timestamp drift error occurs when the difference between OLT's and ONU's clocks exceeds some predefined threshold. This condition can be independently detected by the OLT or an ONU. The OLT detects this condition when an absolute difference between new and old RTT values measured for a given ONU exceeds the value of guardThresholdOLT (see 64.2.2.1), as shown in FIG. 64-10. An ONU detects the timestamp drift error condition when absolute difference between a timestamp received in an MPCPDU (Multi-Point Control Protocol Data Unit) and the localTime counter exceeds guardThresholdONU (see 64.2.2.1), as is shown in FIGS. 64-10 and 64-11”; or        no control message has been received by the ONU (GATE message) or by the OLT (REPORT message) for more than a predefined duration, called “mpcp_timeout”, which can be equal to one second (second condition).        
These two conditions mean that if a GATE or REPORT message exchange occurs at least every second without clock drift between an ONU and the OLT, the ONU remains in registered state.
If not, the ONU transitions to a deregistered state and a complete discovery procedure has to be started over to re-register the ONU.
In order to avoid detection of timestamp drift error condition during operating of the trunk protection scheme, an intermediate state between the registered state and the deregistered state has been introduced in IEEE1904.1 specification.
In Holdover state, the ONU freezes the state of any timeout watchdogs (in particular the timeout watchdog dedicated to the detection of the timestamp drift error condition) and sets the timeout watchdogs to their default values (initial values) when leaving the Holdover state back to the registered state. The ONU does not transmit any upstream data when the ONU is in the Holdover state. Thus, all pending transmission grants (allocated via GATE messages) to a given ONU are purged upon transition in the Holdover state by the ONU, and the ONU waits for new transmission once the transition from the Holdover state to the registered state is completed. The back-up OLT is then in charge of providing new transmission grants to the ONU.
Thus, the Holdover state does not enable any continuity in upstream transmission as the ONU in the Holdover state can not transmit any data in upstream.
There is a need to enable a protection that is common for trunk and tree schemes and that is transparent from the point of view of the ONUs.